1. Field of the Invention
This invention relates to resistance detection and, more particularly, to the provision of delayed hysteresis in detecting resistance thresholds.
2. Description of the Prior Art
In many communication systems it is necessary to detect the resistance of a cummunication circuit. In telephone subscriber loops, for example, voice frequency gain and supervisory signal enhancement are necessary for extremely long subscriber loops. Facilities for providing such gain and enhancement, called "range extenders with gain (REGs)," are connected to telephone subscriber loops which are so long that the standard supervisory signals require enhancement and the voice frequency signals require amplification. One such range extender is shown in K. E. Stiefel U.S. Pat. No. 4,056,688, granted Nov. 1, 1977.
The length of a telephone subscriber loop is normally measured in ohms of resistance since the need for range extension depends on both the physical length and the gauge of the subscriber loop wires. In order to detect loop resistance in the presence of changing voltage levels on the loop conductors, it is desirable to detect both the loop current and the loop voltage and to combine these signals so as to provide a measure of loop resistance. A resistance detector of this type is shown in the copending application of K. E. Stiefel Ser. No. 85,025, filed of even date herewith.
In such applications, it is necessary to detect when the loop resistance exceeds (or falls below) a particular threshold. When the resistance of the loop has a value which is close to that threshold, it is possible for an oscillatory condition to be created where slight variations in the detected loop resistance (due to temperature changes, for example) rise above and fall below the selected threshold. Since voice frequency gain and supervisory signal enhancement are inserted into the circuit in response to threshold crossings, such oscillatory conditions create undesirable conditions on the loop.
The above-described oscillatory condition can be avoided by providing hysteresis in the resistance detectors. That is, the threshold at which the detector responds can be changed as the detector passes through the threshold. With such an arrangement, once the initial threshold is exceeded by a loop having a resistance near the threshold value, that threshold value is reduced (or increased) so that the detector will remain operated until the resistance value changes to a different threshold. This kind of hysteresis can be provided instantaneously with a simple feedback circuit from the output of the detector to its input. Unfortunately, however, such a rapid change in the threshold of detection causes a response to undesirable, brief changes in the loop current or loop voltage such as are caused by overshoot or temporary noise peaks on the loop.